Design and Implementation of Three-Channel Drainage Pipeline Ground Penetrating Radar Device

In order to solve the current problems that conventional video inspection can only detect, as an internal pipeline defect and drainage pipeline radar inspection device detects in a single direction and at radar frequency in water pipeline defect detection, a three-channel drainage pipeline ground penetrating radar (GPR) inspection device was designed and developed, the assembly and commissioning of the device prototype were completed, and an actual engineering test application was carried out. Focusing on the problem that the detection direction and depth of the single-channel detection device are limited, a three-channel drainage pipeline GPR inspection device is designed to realize the synchronous detection of the inside of the pipeline, the pipeline body, and the external environment of the pipeline, improving the detection depth and efficiency. According to the design scheme of the three-channel drainage pipeline GPR inspection device, the assembly of the device prototype was completed. The device contains three radar channels, the top of the main frequency of the antenna is 1.4 GHz, the two sides are 750 MHz, the video camera has a pixel count of 4 million, and the positioning accuracy is less than 1 mm, the waterproof grade is IP68, the detection accuracy of pipe deformation (slope) is 0.1°, the detection depth outside the pipe is 1.2 m, and the detection accuracy of corrosion thickness is 15 mm. In a practical application of the device, the Jianguomenqiao sewage pipeline in Beijing, China, was tested, resulting in the discovery of 87 defects, including 39 loose soil areas at the bottom of the pipe exterior, 40 void areas, and 8 cavities.


Introduction
The urban underground drainage network is widely distributed. The leakage and damage to an underground drainage pipeline will lead to ground collapse, and the blockage of pipeline sludge will lead to urban drainage blockage and urban waterlogging, threatening the safety of urban operations [1]. Drainage pipes in municipal underground pipelines are often laid in the lowest layers of the ground, and broken and leaky drainage pipes take away the surrounding soil, leaving voids around the pipes and causing upper roads to collapse [2]. Therefore, inspection of drainage pipe quality and voids is essential to combat urban road subsidence.
In response to the above problems, many scholars have carried out a lot of research on the detection techniques and equipment for the urban underground in recent years. closed-circuit television (CCTV) technology to detect defects on the internal surface of pipes has been used for more than 40 years [3]. It is mainly used in sewers and storm water pipes. CCTV mounted on a robot inside a crawler pipe allows the inspector to detect any structural defects by analyzing the returned video and recording the defects through a snapshot of the internal surface [4].
The main conventional in-pipe video inspection equipment currently available is the S300 pipeline image inspection robot from SROD company in China [5], which uses a

Structural Design of Three-Channel Drainage Pipeline GPR Inspection Device
Conventional drainage pipe inspection equipment only has video sensors, which can only be used for internal pipe defect detection; there are video sensors combined with single-channel ground-penetrating radar detection equipment, which has limited coverage and depth of detection. In order to solve this problem, the research and development of the three-channel drainage pipeline ground-penetrating radar device was carried out, and three channels were used for operation in three different directions of the drainage pipeline to realize the synchronous detection of the interior of the drainage pipeline, the pipeline body, and the external environment of the pipeline.
The three-channel drainage pipeline GPR inspection device includes a waterproof pipeline robot device, a GPR detection device, an embedded acquisition and control system, an embedded interface circuit, and a host computer, as shown in Figure 1.

Structural Design of Three-Channel Drainage Pipeline GPR Inspection Device
Conventional drainage pipe inspection equipment only has video sensors, which can only be used for internal pipe defect detection; there are video sensors combined with single-channel ground-penetrating radar detection equipment, which has limited coverage and depth of detection. In order to solve this problem, the research and development of the three-channel drainage pipeline ground-penetrating radar device was carried out, and three channels were used for operation in three different directions of the drainage pipeline to realize the synchronous detection of the interior of the drainage pipeline, the pipeline body, and the external environment of the pipeline.
The three-channel drainage pipeline GPR inspection device includes a waterproof pipeline robot device, a GPR detection device, an embedded acquisition and control system, an embedded interface circuit, and a host computer, as shown in Figure 1.  The waterproof pipeline robotics unit runs inside the pipe to detect defects in the pipe. It is mainly composed of a robot body, a robot arm, and a robot walking system. The robot arm adopts an adaptive design to meet the detection needs of complex conditions such as deformation in the pipeline.
Drainage pipes in China are currently dominated by concrete pipes. As the pipes get older, microorganisms in the sewage inside the pipes produce hydrogen, which causes corrosion of the concrete pipes. As corrosion is usually most severe at the top of the pipeline, GPR technology is used to detect the depth of corrosion and determine defects in the pipeline. The depth and accuracy of detection depends on the frequency of the GPR antenna, as shown in Table 1.  The waterproof pipeline robotics unit runs inside the pipe to detect defects in the pipe. It is mainly composed of a robot body, a robot arm, and a robot walking system. The robot arm adopts an adaptive design to meet the detection needs of complex conditions such as deformation in the pipeline.
Drainage pipes in China are currently dominated by concrete pipes. As the pipes get older, microorganisms in the sewage inside the pipes produce hydrogen, which causes corrosion of the concrete pipes. As corrosion is usually most severe at the top of the pipeline, GPR technology is used to detect the depth of corrosion and determine defects in the pipeline. The depth and accuracy of detection depends on the frequency of the GPR antenna, as shown in Table 1. The three-channel GPR inspection device is composed of three channels, the angle between the three channels is 120 • , and each channel is composed of a video camera system and an antenna system. The tube top radar antenna system is the first channel, the GPR antenna main frequency is 1.4 GHz for corrosion depth and wall thickness inspection of the pipe tops, the left tube bottom radar antenna system is the second channel, the right tube bottom radar antenna system is the third channel, the main frequency of the left tube and right tube antenna is 750 MHz for the detection of external defects on the bottom of the tube, as shown in Figure 2. The three-channel GPR inspection device is composed of three channels, the angle between the three channels is 120°, and each channel is composed of a video camera system and an antenna system. The tube top radar antenna system is the first channel, the GPR antenna main frequency is 1.4 GHz for corrosion depth and wall thickness inspection of the pipe tops, the left tube bottom radar antenna system is the second channel, the right tube bottom radar antenna system is the third channel, the main frequency of the left tube and right tube antenna is 750 MHz for the detection of external defects on the bottom of the tube, as shown in Figure 2.

Three-Channel Drainage Pipeline GPR Hardware Design
The three-channel drainage pipeline GPR control system is composed of a GPR detection device and an embedded acquisition control system. It is composed of a radar acquisition control module, a gyroscope module, three radar transmitters, three radar receivers, three arc-shaped transceiver antennas, and a power supply module, as shown in Figure 3.

Three-Channel Drainage Pipeline GPR Hardware Design
The three-channel drainage pipeline GPR control system is composed of a GPR detection device and an embedded acquisition control system. It is composed of a radar acquisition control module, a gyroscope module, three radar transmitters, three radar receivers, three arc-shaped transceiver antennas, and a power supply module, as shown in Figure 3.  The three-channel drainage pipeline GPR acquisition and control system adopts the embedded design of ethernet technology. The system is based on the client/server (C/S) network architecture, and the network transmission protocol is the UDP protocol. The lower computer, the server, uses the Arm Cortex-M7 chip as the core to build an embedded acquisition and transmission platform. The server uses the RT Thread embedded realtime operating system. The application program is based on the embedded lightweight TCP/IP protocol (LwIP) protocol stack RAM interface development. The controller uses the data transceiver to collect and convert radar transmitted wave signals to realize the transmission of radar data and control commands.
The radar acquisition control module is composed of a main control microcontroller unit (MCU) chip, an analog-to-digital (A/D) conversion circuit, a digital-to-analog (D/A) conversion circuit, and a step delay circuit.
The radar acquisition control module is composed of a radar transmitter and a radar receiver. The radar antenna converts electromagnetic wave signals into electrical signals. The radar transmitter is used to generate electromagnetic wave pulses with extremely short rise times, and consists of a regulated power supply, a multi-stage avalanche circuit, a trigger pulse forming circuit, and a high-voltage module. The radar receiver is used to receive electromagnetic wave signals, and is composed of a trigger pulse forming circuit, a low-frequency avalanche circuit, a double pulse generator, a high-frequency sampling head, a high-frequency amplifier, an integrator, and a feedback circuit.
The gyroscope module adopts the MPU9250 gyroscope, which is a six-axis integrated motion processing component, which can avoid the inter-axis difference when combining the gyroscope and the accelerator and saves packaging space. The radar antenna detects the angle by using the gyroscope module to obtain the attitude information. The MPU9250 The three-channel drainage pipeline GPR acquisition and control system adopts the embedded design of ethernet technology. The system is based on the client/server (C/S) network architecture, and the network transmission protocol is the UDP protocol. The lower computer, the server, uses the Arm Cortex-M7 chip as the core to build an embedded acquisition and transmission platform. The server uses the RT Thread embedded real-time operating system. The application program is based on the embedded lightweight TCP/IP protocol (LwIP) protocol stack RAM interface development. The controller uses the data transceiver to collect and convert radar transmitted wave signals to realize the transmission of radar data and control commands.
The radar acquisition control module is composed of a main control microcontroller unit (MCU) chip, an analog-to-digital (A/D) conversion circuit, a digital-to-analog (D/A) conversion circuit, and a step delay circuit.
The radar acquisition control module is composed of a radar transmitter and a radar receiver. The radar antenna converts electromagnetic wave signals into electrical signals. The radar transmitter is used to generate electromagnetic wave pulses with extremely short rise times, and consists of a regulated power supply, a multi-stage avalanche circuit, a trigger pulse forming circuit, and a high-voltage module. The radar receiver is used to receive electromagnetic wave signals, and is composed of a trigger pulse forming circuit, a low-frequency avalanche circuit, a double pulse generator, a high-frequency sampling head, a high-frequency amplifier, an integrator, and a feedback circuit.
The gyroscope module adopts the MPU9250 gyroscope, which is a six-axis integrated motion processing component, which can avoid the inter-axis difference when combining the gyroscope and the accelerator and saves packaging space. The radar antenna detects the angle by using the gyroscope module to obtain the attitude information. The MPU9250 angular velocity full grid sensing range is ±250, ±500, ±1000, and ±2000 • /s, which can accurately track fast and slow motions. The accelerometer sensing range is controllable, the sensing range is ±2 g, ±4 g, ±8 g, and ±16 g.
The photoelectric conversion module is designed based on a full gigabit fiber optic transceiver, with two 10/100/1000 M adaptive network ports and two 1.25 G uplink optical ports. All ports adopt non-blocking and full line-speed data packet forwarding. The network port supports automatic MDI/MDIX identification and flipping, has adaptive full-duplex/half-duplex function, supports 2048-byte frames, and supports a maximum of 10 Kbyte ultra-long data packets. The network transmission distance is extended from the limit distance of 100 m of copper wire to 120 km. The optical port single-mode wavelength is 1310/1550 nm, and the DFB wavelength is 1260~1610 nm.
The power module adopts the URA_YMD-15WR3 module and the URB_YMD-15WR3 module. The URA_YMD-15WR3 module can convert the external power supply 48 V to +12 V and −12 V. The URB_YMD-15WR3 module can convert the external power supply 48 V to +5 V.
The power supply module supplies power to the radar acquisition control module, and the main control MCU chip of the radar acquisition control module transmits the radar signal to the host computer through the photoelectric conversion module. The trigger signal of the radar transmitter and the trigger signal of the radar receiver are generated by a DA conversion circuit, and the A/D conversion circuit collects the radar signal received by the radar receiver. The gyroscope module collects the detection angle of the radar antenna and transmits it to the host computer through the radar acquisition control module. The host computer combines the data to calibrate the radar detection angle to improve the accuracy of the detection results.

Overall Design of GPR Data Acquisition for Three-Channel Drainage Pipeline
The overall process of GPR data acquisition for three-channel drainage pipes is shown in Figure 4. The upper computer serves as the sending end to send commands to the lower computer at the receiving end; after the lower computer receiving end obtains the command information of the upper computer, it needs to analyze it. There are two types of modes: the single-channel sampling mode and the continuous multichannel sampling mode designed in this area, and the single-channel buffer or multichannel sampling circular queue buffer is selected. According to the single sampling and continuous sampling commands, the sampling process is executed, and finally, the collected channel number information is sent to the host computer to complete the data sending process.

Prototype Assembly of Three-Channel Drainage Pipeline GPR Inspection Device
According to the design drawing of the three-channel drainage pipeline GPR inspection device (as shown in Figure 2), the assembly of the three-channel drainage pipeline detection device is completed, and the model machine photo is shown in Figure 5. The three sides of the main body of the three-channel drainage pipeline GPR inspection device are connected to the receiving and transmitting antenna box, the plastic upper cover of the antenna box is fixed on the metal plate at its bottom with screws, the radiation surface and absorbing material of the GPR transmitting and receiving antenna are

Prototype Assembly of Three-Channel Drainage Pipeline GPR Inspection Device
According to the design drawing of the three-channel drainage pipeline GPR inspection device (as shown in Figure 2), the assembly of the three-channel drainage pipeline detection device is completed, and the model machine photo is shown in Figure 5.

Prototype Assembly of Three-Channel Drainage Pipeline GPR Inspection Device
According to the design drawing of the three-channel drainage pipeline GPR inspec tion device (as shown in Figure 2), the assembly of the three-channel drainage pipeline detection device is completed, and the model machine photo is shown in Figure 5. The three sides of the main body of the three-channel drainage pipeline GPR inspec tion device are connected to the receiving and transmitting antenna box, the plastic uppe cover of the antenna box is fixed on the metal plate at its bottom with screws, the radiation surface and absorbing material of the GPR transmitting and receiving antenna are The three sides of the main body of the three-channel drainage pipeline GPR inspection device are connected to the receiving and transmitting antenna box, the plastic upper cover of the antenna box is fixed on the metal plate at its bottom with screws, the radiation surface and absorbing material of the GPR transmitting and receiving antenna are contained in the plastic upper cover, the metal plate at the bottom of the antenna box and the corresponding position of the main body are installed with a limited base, the metal rod is used to form the rocker to connect the two parts, there are two plastic directional wheels on both sides of the metal plate at the bottom of each antenna box, and a three-side directional wheel forms the fulcrum and attaches to the inner wall of the pipe to make the main body move in the direction of the pipe. A U-shaped metal handle is installed on one side of the main body, and steel wire rope is installed on the camera side, which is fixed to the main body with screws for traction and handling.
The support wheels of the curved antennas on the left and right sides of the threechannel drainage pipe GPR inspection device are McNamee wheels that can rotate omnidirectionally, allowing the robot to spiral forward in the pipe and thus measure the GPR signal in the circumferential direction. The top curved antenna is fitted with a guide wheel structure in the forward direction. When the robot rotates itself during the inspection process, the internal inertial navigation sensor acquires the angle of rotation and the master control chip controls the reverse rotation of the guide wheel, allowing the PPR to correct the deflection, thus ensuring the accuracy of the inspection angle.
The main body of the three-channel drainage pipeline GPR inspection device includes the acquisition control circuit, camera, radar receiver, radar transmitter, etc.

Positioning Accuracy
In order to achieve high-precision mileage positioning of the pipeline robot, the 1000BZ-05CG21 pulse encoder is used to sample its mileage information. The circumference of the ranging wheel is 330 mm, the number of pulses is 1000, and the maximum mileage positioning accuracy is 0.33 mm. As the robot travels through the tube, the distance measuring wheel and encoder, which are fixed to the line carriage, are rotated by a transmission cable to obtain mileage information.

Sensitivity Test of the Gyroscope Linear Acceleration
In order to realize the high-precision spatial positioning and attitude confirmation of the pipeline robot, the MPU9250 electronic gyroscope is used to sample its linear acceleration, with a three-axis angular velocity sensor of ±2000 • /s and a three-axis accelerator of ±16 g. The sensitivity of the linear acceleration of the gyroscope is 0.06 • /s/g.

Video Camera Pixel Test
The video camera uses a SONY VRS-MH8100 high-definition camera, 1/3 CMOS, 10.5 times optical zoom, the effective pixel is 4.08 million, and the camera resolution of this model is 1080p full HD.

Slope Test Index
The slope is obtained from the pipeline model, and then the slope estimation parameters at different locations are obtained. The slope of the pipeline model is −3.25 • , the actual test slope is between −3.13 • and −3.35 • , and the slope accuracy is 0.1 • .

Radar Detection Index Test
The number of channels of the test device is three. The detection depth test is carried out in a known pipeline, and the 400-600 mm pipeline robot detection image is obtained. The depth of the cavity detection is more than 1.2 m, as shown in Figure 6. It indicates that the detection depth can reach 1.2 m. This three-channel drainage pipeline GPR device is dedicated for use in pipe defect detection. To illustrate the performance of this system, the comparison of the key parameters between this system and the other GPR systems (ACPS), which can be used to detect defects in 400~600 mm drainage pipes, is shows in Table 2.

Three-Channel Drainage Pipeline GPR Detection Technology Application
Based on the developed three-channel drainage pipeline GPR inspection device, an experimental application of urban drainage pipeline defect detection project was carried out. The following is the test case of Jianguomenqiao sewage pipeline in Beijing, China.

Detection Purpose and Detection Location
The purpose of this inspection is to inspect the structural and functional conditions of the drainage pipeline, including the soil anomalies, cracks, bulges, corrosion, and other defects around the pipeline, as well as the pipeline joints, pipeline deflection, and other conditions. The inspect standards are based on Beijing Drainage Group's corporate stand- The test area is located at Jianguomen Bridge, Beijing, China. The observation line is located almost in the northerly direction. There are eight pipe sections on the observation This three-channel drainage pipeline GPR device is dedicated for use in pipe defect detection. To illustrate the performance of this system, the comparison of the key parameters between this system and the other GPR systems (ACPS), which can be used to detect defects in 400~600 mm drainage pipes, is shows in Table 2.

Three-Channel Drainage Pipeline GPR Detection Technology Application
Based on the developed three-channel drainage pipeline GPR inspection device, an experimental application of urban drainage pipeline defect detection project was carried out. The following is the test case of Jianguomenqiao sewage pipeline in Beijing, China.

Detection Purpose and Detection Location
The purpose of this inspection is to inspect the structural and functional conditions of the drainage pipeline, including the soil anomalies, cracks, bulges, corrosion, and other defects around the pipeline, as well as the pipeline joints, pipeline deflection, and other conditions. The inspect standards are based on Beijing Drainage Group's corporate

Data Acquisition Process
The three-channel drainage pipeline radar detection device is used for pipeline defect detection. The number of antenna channels is three, the antenna frequency at the top is 1.4 GHz, and the antenna frequency at both sides is 750 MHz. One CCTV industrial camera is used. The 1.4 GHz radar antenna is used to detect the corrosion depth and wall thickness at the top of the pipeline with a sampling time window of 10 ns. A 750 MHz radar antenna is used to detect voids on the outside of the pipeline with a sampling time window of 30 ns. The instrument is triggered by a high-precision rangefinder wheel in the line trolley with a trace interval of 0.5 mm.
The acquisition process of the three-channel drainage pipeline detection device is shown in Figure 8. The data acquisition includes the environmental and geological background investigation of the construction site, instrument installation and commissioning, survey line layout, parameter setting of the radar detection system for the three-channel drainage pipeline, and field parameter commissioning. It is worth noting that the antenna should be as close to the inner wall of the pipeline as possible during the detection. Figure 8. The acquisition process of the three-channel drainage pipe inspection.

Drainage Pipeline Defect Detection Results
A total of 87 defects were found during the drainage pipeline defect detection, including 39 loose soil areas at the bottom of the pipeline, 40 void areas, and 8 cavities.

Data Acquisition Process
The three-channel drainage pipeline radar detection device is used for pipeline defect detection. The number of antenna channels is three, the antenna frequency at the top is 1.4 GHz, and the antenna frequency at both sides is 750 MHz. One CCTV industrial camera is used. The 1.4 GHz radar antenna is used to detect the corrosion depth and wall thickness at the top of the pipeline with a sampling time window of 10 ns. A 750 MHz radar antenna is used to detect voids on the outside of the pipeline with a sampling time window of 30 ns. The instrument is triggered by a high-precision rangefinder wheel in the line trolley with a trace interval of 0.5 mm.
The acquisition process of the three-channel drainage pipeline detection device is shown in Figure 8. The data acquisition includes the environmental and geological background investigation of the construction site, instrument installation and commissioning, survey line layout, parameter setting of the radar detection system for the three-channel drainage pipeline, and field parameter commissioning. It is worth noting that the antenna should be as close to the inner wall of the pipeline as possible during the detection.

Data Acquisition Process
The three-channel drainage pipeline radar detection device is used for pipeline defect detection. The number of antenna channels is three, the antenna frequency at the top is 1.4 GHz, and the antenna frequency at both sides is 750 MHz. One CCTV industrial camera is used. The 1.4 GHz radar antenna is used to detect the corrosion depth and wall thickness at the top of the pipeline with a sampling time window of 10 ns. A 750 MHz radar antenna is used to detect voids on the outside of the pipeline with a sampling time window of 30 ns. The instrument is triggered by a high-precision rangefinder wheel in the line trolley with a trace interval of 0.5 mm.
The acquisition process of the three-channel drainage pipeline detection device is shown in Figure 8. The data acquisition includes the environmental and geological background investigation of the construction site, instrument installation and commissioning, survey line layout, parameter setting of the radar detection system for the three-channel drainage pipeline, and field parameter commissioning. It is worth noting that the antenna should be as close to the inner wall of the pipeline as possible during the detection.

Drainage Pipeline Defect Detection Results
A total of 87 defects were found during the drainage pipeline defect detection, including 39 loose soil areas at the bottom of the pipeline, 40 void areas, and 8 cavities.

Drainage Pipeline Defect Detection Results
A total of 87 defects were found during the drainage pipeline defect detection, including 39 loose soil areas at the bottom of the pipeline, 40 void areas, and 8 cavities.

Void Defect Detection
A void refers to the gap between the outside of the pipe wall and the structural plane. The void area is often small vertically and develops horizontally along the structural plane. The void is often filled with air or rich water.
The typical void defect GPR image is shown in Figure 9. The void interface has strong reflection, which is distributed in an approximate horizontal band with multiple reflection signals. The detected void is located on the left side of the survey line with the pipe section number of 25,822~28,384 and starting and ending mileages of 9.9~11.8 m.

Void Defect Detection
A void refers to the gap between the outside of the pipe wall and the structural plane. The void area is often small vertically and develops horizontally along the structural plane. The void is often filled with air or rich water.
The typical void defect GPR image is shown in Figure 9. The void interface has strong reflection, which is distributed in an approximate horizontal band with multiple reflection signals. The detected void is located on the left side of the survey line with the pipe section number of 25822~28384 and starting and ending mileages of 9.9~11.8 m.

Loose Defect Detection
Looseness refers to a loose soil layer around the pipeline with insufficient bearing capacity. The loose defect is mainly caused by the substandard compactness of the backfill soil when the excavation part is backfilled during the pipeline placement.
A typical loose defect GPR image is shown in Figure 10. The loose interface is a strong reflection, and the isophase axis is discontinuous, staggered, and disordered. The detected looseness is located on the left side of the survey line with the pipe section number of 25822~28384 and starting and ending mileages of 17.5~19.5 m.

Loose Defect Detection
Looseness refers to a loose soil layer around the pipeline with insufficient bearing capacity. The loose defect is mainly caused by the substandard compactness of the backfill soil when the excavation part is backfilled during the pipeline placement.
A typical loose defect GPR image is shown in Figure 10. The loose interface is a strong reflection, and the isophase axis is discontinuous, staggered, and disordered. The detected looseness is located on the left side of the survey line with the pipe section number of 25,822~28,384 and starting and ending mileages of 17.5~19.5 m.

Void Defect Detection
A void refers to the gap between the outside of the pipe wall and the structural plane. The void area is often small vertically and develops horizontally along the structural plane. The void is often filled with air or rich water.
The typical void defect GPR image is shown in Figure 9. The void interface has strong reflection, which is distributed in an approximate horizontal band with multiple reflection signals. The detected void is located on the left side of the survey line with the pipe section number of 25822~28384 and starting and ending mileages of 9.9~11.8 m.

Loose Defect Detection
Looseness refers to a loose soil layer around the pipeline with insufficient bearing capacity. The loose defect is mainly caused by the substandard compactness of the backfill soil when the excavation part is backfilled during the pipeline placement.
A typical loose defect GPR image is shown in Figure 10. The loose interface is a strong reflection, and the isophase axis is discontinuous, staggered, and disordered. The detected looseness is located on the left side of the survey line with the pipe section number of 25822~28384 and starting and ending mileages of 17.5~19.5 m.

Cavity Defect Detection
A cavity refers to the separation between the structural layer and the soil foundation. The scale of the cavity is large, and the soil mass in the internal area of the structural layer is missing.
A typical cavity defect GPR image is shown in Figure 11. The boundary surface of the cavity has a strong reflection, which is a regular or irregular hyperbolic waveform and is characterized by a solitary phase. The number of the detected pipe section is 25,822~28,384, and the starting and ending mileages are 52.7~53.3 m, as located on the right side of the survey line.

Cavity Defect Detection
A cavity refers to the separation between the structural layer and the soil foundation. The scale of the cavity is large, and the soil mass in the internal area of the structural layer is missing.
A typical cavity defect GPR image is shown in Figure 11. The boundary surface of the cavity has a strong reflection, which is a regular or irregular hyperbolic waveform and is characterized by a solitary phase. The number of the detected pipe section is 25822~28384, and the starting and ending mileages are 52.7~53.3 m, as located on the right side of the survey line.

Conclusions
In this paper, a three-channel drainage pipe radar (GPR) unit for drainage pipe defect detection is designed and tested, incorporating multi-sensor technologies such as closedcircuit television (CCTV) and inertial navigation units (IMU). Compared to other pipeline inspection systems, this system is specifically adapted for pipeline defect detection and has the following design features: three-channel operation effectively improves the survey efficiency, achieving reflection radar data with different frequencies from three angles in one single pass; it can realize the synchronous detection of the interior, body, and external environments of the pipeline.
The experimental application results verify that the design is a relatively fast and effective non-destructive testing method that does not interrupt the operation of the drainage pipe network. The corrosion detection accuracy of the top of the pipe is 15 mm and the detection range of the cavity outside the pipe is 1.2 m, solving the problem of limited detection angle and depth.
For the next step of development, we will investigate situations in which the device may slip or rotate during the inspection process inside the pipe, and improve the control algorithm to correct its own rotation to improve the accuracy of the detection angle of this device.
Author Contributions: M.X. contributed to the research idea, methodology, and model design. He also wrote the main portion of the manuscript. F.Y. proposed the correction conceptualization and validated the whole integration scheme, and was responsible for the thesis revision proofreading. R.Y. assisted with three-channel drainage pipeline ground penetrating radar (GPR) inspection device data acquisition, and analysis. All authors have read and agreed to the published version of the manuscript.

Conclusions
In this paper, a three-channel drainage pipe radar (GPR) unit for drainage pipe defect detection is designed and tested, incorporating multi-sensor technologies such as closedcircuit television (CCTV) and inertial navigation units (IMU). Compared to other pipeline inspection systems, this system is specifically adapted for pipeline defect detection and has the following design features: three-channel operation effectively improves the survey efficiency, achieving reflection radar data with different frequencies from three angles in one single pass; it can realize the synchronous detection of the interior, body, and external environments of the pipeline.
The experimental application results verify that the design is a relatively fast and effective non-destructive testing method that does not interrupt the operation of the drainage pipe network. The corrosion detection accuracy of the top of the pipe is 15 mm and the detection range of the cavity outside the pipe is 1.2 m, solving the problem of limited detection angle and depth.
For the next step of development, we will investigate situations in which the device may slip or rotate during the inspection process inside the pipe, and improve the control algorithm to correct its own rotation to improve the accuracy of the detection angle of this device.
Author Contributions: M.X. contributed to the research idea, methodology, and model design. He also wrote the main portion of the manuscript. F.Y. proposed the correction conceptualization and validated the whole integration scheme, and was responsible for the thesis revision proofreading. R.Y. assisted with three-channel drainage pipeline ground penetrating radar (GPR) inspection device data acquisition, and analysis. All authors have read and agreed to the published version of the manuscript.